The study also looked at cities considered "underbanked" -- where even people who do have accounts continue to rely more heavily on alternative financial services.

Those services include check-cashing shops, payday loans, or pawn shops.

The top underbanked city is Detroit, with 29.3 percent; followed by Memphis, with 28.1 percent; Dallas, 26 percent; El Paso, 25.7 percent; Houston, 25.6 percent; and Cleveland and Baltimore, tying at 25.4 percent.

The Miami Herald reports that those without bank accounts include people who are "unemployed, low-income workers with a shortage of cash, and immigrants who may not trust financial service providers."

To reach underbanked residents, banks must strive to gain their trust.

"If you want to change the fabric of consumer finance in America, you have to change consumer behavior in some way. And it's very hard to change consumer behavior if people don't trust you," said Raj Date, managing partner of business-consulting firm Fenway Summer, according to the Herald.